Author: Rajesh Bhaskaran & Yong Sheng Khoo, Cornell University Problem Specification |
If you are having trouble following written tutorial, detailed video tutorials are available here |
In an external flow such as the flow past a cylinder, we have to define farfield boundaries and mesh the region between the cylinder geometry and the boundaries. Farfield boundaries should be placed well away from the cylinder such that the boundary conditions will not affect the flow near cylinder.
Figure above shows the geometry of such case.
To model this flow, we need a cylinder and farfield boundaries. We need finer meshes around the cylinder to capture the active region (call this radius of influence). Downstream of the flow, there will be wake generated by the cylinder, which requires finer mesh to better capture this phenomena. To be able to specify such regions, we split the model into different faces as shown below.
We set the geometry upstream to be shorter because we have less activity before flow through cylinder. We set the geometry downstream of the cylinder to be relatively longer because we want to make sure that the downstream farfield boundary is not affected by the wake generated.
Create a folder called cylinder in a convenient location. We'll use this as the working folder in which files created during the session will be stored.
Create a new directory called cylinder and start GAMBIT from that directory by typing gambit -id cylinder at the command prompt.
Under Main Menu, select Solver > FLUENT 5/6 since the mesh to be created is to be used in FLUENT 6.0.
Create the cylinder using two arc. First arc span from -90 to 90 deg. Second arc span from 90 to -90 deg. Both arc with radius 1.
Operation Toolpad > Geometry Command Button > Edge Command Button > Create Edge > Arc >
Create Radius of InfluenceNow we can proceed to create the geometry for radius of influence. Since both the cylinder and radius of influence is of same shape. We can create the radius of influence by copying and scaling up the cylinder. Operation Toolpad > Geometry Command Button > Edge Command Button > Move/Copy Edges The current geometry should look like this:
editor NOTE (duplicate vertices created...)
Create Farfield BoundariesCreate Upstream Boundary We will first create the semi-circle upstream boundary. Operation Toolpad > Geometry Command Button > Edge Command Button > Create Edge > Arc > Input the following data and click Apply.
Create Downstream Boundaries In creating downstream boundaries, we will use bottom up approach where we first create vertices and join the vertices to form edges. Create vertices with following coordinates:
Operation Toolpad > Geometry Command Button > Vertex Command Button > Create Vertex Create the vertices by entering the coordinates under Global. Click the FIT TO WINDOW button to scale the display so that you can see all the vertices.
Connect AB, BC and CD to form three edges. Operation Toolpad > Geometry Command Button > Edge Command Button > Create Edge
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You should see have this geometry up till this step.
We would like to have more mesh elements at the downstream of the flow, to the right of the cylinder. To accomplish this, we would have to split some edges.
Split the edge according to the figure shown below:
First split the straight edges
Label |
x |
y |
z |
E |
2.83 |
10 |
0 |
H |
2.83 |
-10 |
0 |
I |
40 |
2.83 |
0 |
J |
40 |
-2.83 |
0 |
Select appropriate edge one by one. For example, to create vertex H, we have to split the bottom edge.
Operation Toolpad > Geometry Command Button > Edge Command Button > Split Edge
And then select the bottom edge. Enter the coordinate value for vertex H. Do this for the rest of vertices.
Now, we will split the curved edges
Label |
r |
t |
z |
F |
4 |
45 |
0 |
G |
4 |
-45 |
0 |
Operation Toolpad > Geometry Command Button > Edge Command Button > Split Edge
Change the coordinate Type to Cylindrical and create vertex point F and G.
Finally, connect all the remaining vertices KL, LM, NO, OP, FI and GJ.
The newly created vertices to form two new edges. The current geometry in Gambit should look like this:
We can now join all the edges to form faces.
Operation Toolpad > Geometry Command Button > Face Command Button > Form Face
This brings up the Create Face From Wireframe menu. Recall that we had selected vertices in order to create edges. Similarly, we will select edges in order to form a face.
There will be total of six faces.
Create all the six faces by connecting appropriate edges.
We are now ready to mesh the geometry.
Go to Step 2: Mesh Geometry in GAMBIT
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